Water-Soluble Ruthenium Alkylidenes: Synthesis, Characterization, and Application to Olefin Metathesis in Protic Solvents

Abstract

Ruthenium alkylidenes 6 and 7 bearing cationically functionalized phosphine ligands are soluble and stable in protic solvents and initiate olefin metathesis reactions in methanol, water, and aqueous emulsions. NMR spectroscopy data and X-ray diffraction analyses of alkylidene 6 suggested that these new alkylidenes were structurally similar to previously reported complexes of the type (PR_3)_2Cl_2Ru ═ CHR, in which the alkylidene substituents lie in the Cl−Ru−Cl plane. The anionic chloride ligands of complexes 6 and 7 were found to undergo facile ligand exchange reactions with other anions in protic solution. Both alkylidenes initiate the ring-opening metathesis polymerization (ROMP) of strained, cyclic olefins in water. However, the propagating species in these reactions decompose prior to complete consumption of monomer. These complexes initiate the quantitative, living polymerization of functionalized monomers in water in the presence of a Brønsted acid. Both chain termination and chain transfer reactions were experimentally demonstrated to be absent on the time scale of these acid-activated polymerizations. Alkylidenes 6 and 7 react readily with acyclic olefins:  alkylidene 6 reacted with trans-2-butene to yield a new ethylidene complex in either methanol or water, and the treatment of 6 with tri(ethylene glycol) methyl vinyl ether in water yielded water-soluble Fischer−carbene complex 16.